Desmond wrong or lying?

Rewatching old seasons is great in light of new information! (I'm getting close to meeting Daniel for the 1st time & am anxious to see if we EVER see his neck -- or his chest for that matter! He may be the only dude who never loses his shirt, and in fact he wears a TIE on a tropical island!?! -This just can't be coincidence, can it? Sawyer even demanded his shirt once & he changed the subject immediatelly.)

I'd like some opinions on a question I have: [Greatest Hits: 3x21]
The thing that convinces Charlie to overcome his fear of death and actually volunteer is because Desmond told him he saw a flash of Claire & Aaron getting on a rescue helicopter.

Firstly, why/how did Desmond know that Charlie flipping the switch would be the catalyst for Claire/Aaron's rescue? Does it seem his flashes should work like that? (Seeing Charlie die & Claire get rescued does not show cause & effect.)

My primary question though is this: Which of these is true?: (or are there other options?)
(1) Desmond was lying about his vision of Claire & Aaron getting on the chopper.
(2) Desmond was simply wrong about the Claire part (maybe he mistook Kate for Claire).
(3) Something changed the future. All of Desmond's other flashes seemed inevitable -- they could be put off for a very short time, but were definitely going to happen. --I don't see Claire & Aaron getting on a chopper any time soon.